Polarization insensitive microwave energy phase shifter



y 1969 E. J. SHELDON 3,445,851

POLARIZATION INSENSITIVE MICROWAVE ENERGY PHASE SHIFTER Filed Sept. 16, 1966 Sheet of 2 22 2 BEAM DIRECTION SH|FTER TRANSMITTER l6 RECEIVER l INPUT FROM FEED HORNS R1 AFTER SHORT cmcurr :36 A; '//////l g/ REFLECTION 34 k E L 26 W 40 F/G 2 I 42 7 V 38 l I LHP PHASE DELAY 10 PHASE ADVANCE R H P A INVENTOR EDWARD J. SHE]. DON

May 20, 1969 E. J. SHELDON 3,445,351

POLARIZATION INSENSITIVE MICROWAVE ENERGY PHASE SHIFTER Filed Sept. 16, 1966 Sheet 3 of 2 lNl/E/VTOR EDWARD J. SHELDON ATTORNEY 3 445 851 POLARIZATIGN IN SEf iSlTIVE MICROWAVE ENERGY PHASE SHIFTER Edward J. Sheldon, Lexington, Mass, assignor to Raytheon Company, Lexington, Mass, a corporation of Delaware Filed Sept. 16, 1966, Ser. No. 579,914 Int. Cl. H01q 19/06 US. Cl. 343-754 8 Claims The present invention relates generally to microwave energy phase shifting devices and more specifically to a passive analog ferrite phase shifter that will simultaneously support orthogonal polarizations of the electric field vectors in a phased array antenna system.

In the transmission and detection of long range radar signals, electrically scanned antennas have in recent years achieved wide usage in view of the rapid scanning rates attainable without the use of any mechanical components. Planar antennas of considerable height and length utilize a large number of individual elements to provide variable electrical length to collimate and steer high power electromagnetic wave energy in a predetermined wave front. Ferrite phase shifters utilizing the Faraday rotation effect are commonly employed to establish the controlled phase relationship of the radiated wave front in such systems where power levels are in excess of the limits tolerable by semiconductor or mechanical variable phase shifters. Such devices commonly consist of a longitudinally magnetized rod of a ferromagnetic material axially positioned within a hollow conductive pipe or waveguide structure and parallel to the direction of Wave propagation. The electric vectors of the electromagnetic waves can be phase shifted as well as delayed in time by varying the relative strength of the applied unidirectional magnetic field. As the electric current to the field producing means is varied the microwave permeability of the ferrite material is correspondingly varied to make the device electrically longer or shorter to delay or advance the propagated waves. The onolinearity of ferrite materials when operated at or below ferromagnetic resonance is the parameter considered in the application of such materials in microwave devices.

In the past ferrite phase shifters have been coupled to individual microwave transmission elements with the beam direction determined by a central computer programmer to establish the proper phase of the wave front. Each antenna element is coupled by a complex corporate structure type network to a high power microwave energy source and may be referred to in the art as a transmission type phased array antenna system. In the co-pending patent application, Ser. No. 322,008, filed Nov. 5, 1963, now Pat No. 3,305,867 by Aldo R. Miccioli et a1. entitled Antenna Array System and assigned to the assignee of the present invention, a new concept in phased array antennas is disclosed wherein a large array of individual antenna elements, preferably passive, is optically fed from a physically and conceptually separate radiant energy generation source. Each of the passive elements in the array include phase shifter means together with the radiating element to collectively define the beam steering elements. The high power energy generator and transmitter can be designed to provide the desired peak and average powers in the most econimocal way possible and since the energy is transmitted through free space to illuminate the phased array antenna system, thousands of transmission lines required in prior art array antennas are eliminated. The antenna system is correspondingly lighter by the elimination of the individaully coupled transmitters. The disclosed optically fed antenna array system referred to in the referenced application is also 3,445,851 Patented May 20, 1969 capable of being utilized for both transmission and reception with the duplexing accomplished in the conventional manner by a single high power antenna horn and a transducer mechanism to switch the horn to a receive mode after the transmission cycle. This antenna array system is commonly referred to as the reflector type and the beam steering elements provide for the traversal of the energy in two directions and hence are reentrant devices.

I Such reflector type systems commonly employ passive analog phase shifters which preferably propagate circularly polarized electromagnetic energy. For the purposes of this description, an electrical wave is said to be circularly polarized when the field thereof can be resolved vectorially into two orthogonal components of equal amplitude apart in space and 90 apart in time. If the two vectors are not of equal amplitude or if the time difference is other than 90 the wave is considered to be elliptically polarized. Linearly polarized waves are those in which the time phase angle reduces to zero.

Heretofore ferrite phase shifters employed in optically fed phased array antenna systems have been polarization sensitive when the preferred circular polarized waves are utilized. As a result if a signal is transmitted, for example right hand circular, the antenna element after the energy traverses the phase shifter in both directions would radiate left hand circular polarized waves. Such polarization sensitivity limits the utility of the system and leads to reception of undesirable target data such as rain clutter. The term right hand circular polarization is defined as an electromagnetic wave which is propagated in a manner similar to that of the right hand screw so that when traveling away from the observer its observed direction of rotation is clockwise. The term left hand circular polarization will be considered as being in the opposite direction of rotation or counterclockwise.

It is a primary object of the present invention to provide a microwave energy phase shifter for propagating and steering radar signals irrespective of the plane of polarization of the electromagnetic waves.

A further object of the present invention is the provision of a one port reentrant ferrite phase shifter for reflector type optically fed phased array antenna sys tems wherein the phase shift is identical for circular, horizontal or vertical polarized waves and any combinations thereof.

A still further object of the present invention is the operation of a ferrite phase shifter of the Faraday rotator type having inherent nonreciprocal phase-current characteristics in a reciprocal manner to shift the electrical phase of microwave energy having any orthogonal distribution of the electromagnetic field vectors.

Other objects, features and advantages will become readily apparent after consideration of the following detailed description of the invention together with the accompanying drawings, in which:

FIG. 1 is a diagrammatic presentation of a reflector type optically fed phased array antenna system;

FIG. 2 is a diagrammatic presentation and accompanying explanatory matter of a prior art phase shifter employed in reflector type phased array antenna systems;

FIG. 3 is a diagrammatic illustration of the embodiment of the present invention;

FIG. 4 is a graph illustrative of the phase-current characteristics of a ferrite phase shifter propagating two types of circular polarized energy; and

FIG. 5 is an isometric view, partly in section, of an illustrative polarization insensitive ferrite phase shifter according to the present invention.

Referring to the drawings, FIG. 1 is illustrative of a reflector type optically fed phased array antenna system in accordance with the teachings of the co-pending patent application. Reentrant type phase shifters 2 are incorporated in each of the antenna passive beam steering elements which collectively define the array antenna 4. In addition to the phase shifter section radiating elements 6 for receiving the uncollimated energy and emitting the steered beam are provided. A high power electromagnetic wave generator, shown generally as transmitter 8, is disposed at a point distant from the array antenna and the energy is radiated by means of a transmitter horn 10 of any of the well known structures in the art. A circular polarizer 12 may be incorporated at the exit channel of the horn as well as a two mode transducer in order that the over-all system may be utilized in duplexing of transmit and receive signals as will be hereinafter evident. In such operation a suitable receiver 16 will be coupled to the same horn.

The ferrite phase shifters 2 are initially set by adjusting the coil current of the magnetic field producing means so as to provide an effective electrical length across the surface of the array antenna with the coil current supplied by means of leads 18 from the programmer-20. The microwave energy from the transmitter horn 10 is radiated toward the array 4 in a divergent beam designated A in the drawing. Each element of the array is positioned within a circular waveguide channel and terminated at the end by short circuit means 22. The power enters each of the antenna elements through the radiating element 6, traverses the phase shifting section and is reflected from the short circuit to transverse for the second time the phase shifter section and in turn be radiated with the phase determined by the current settings of the individual ferrite phase shifters. The divergent rays emanating from the transmitter horn 10 are collimated and converted into a plane wave front of uniform phase designated by the letter B. Any desired amount of phase shift may be imposed upon the divergent beam by programming the appropriate current through the magnetic field producing means of the individual ferrite phase shifters so as to steer the beam in the desired direction. In the illustration the beam direction is designated C and it is evident that energy originating from this direction will be received by the array antenna, phase shifted and reflected back towards the transmitter horn. Duplexing operation is therefore permissible through the utilization of the two-mode transducer with appropriate switching to couple the receiver apparatus 16 in the system.

One of the advantages of the reflector type optically fed system is that since energy is passed through the ferrite phase shifters twice, the amount of phase shift realized and accompanying adjustment of the over-all electrical length is achieved utilizing approximately one-half the ferrite material required in the so-called transmission type two port systems. It may also be noted that the manner of placement of the transmitter horn is well known to those skilled in the art and may be off-set with the electrical path lengths equalized by suitable adjustment of the currents of the phase shifters within the passive antenna elements. In addition, it may be desirable to utilize the Cassegrainian type optically fed antenna horn which is also considered as well known in the art and not requiring further elaboration in this description.

To achieve an understanding of the polarization insensitive feature of the ferrite phase shifter of the present invention an explanation follows of the nonreciprocal behavior characteristics of a circular polarized phase shifter, reference being directed to FIG. 2. The phase shifter employs a ferrite rod 24 of a predetermined length centrally located within a circular waveguide 26 which is preferred for supporting circularly polarized waves. A longitudinal magnetic field is applied to the rod by means of a solenoid (not illustrated) encircling the waveguide 26 in the manner well known in the art. A unidirectional current and the direction of the windings of the solenoid insure that the direction of magnetization is in the appropriate direction of propagation to achieve the microwave permeability characteristics desired. For a positive current and right hand circular polarization the solenoid windings will encircle the device in a clockwise fashion. Short circuit means 28 by any suitable conductive member at one end of the antenna element provides for the reentrant operation of the device in the array antenna system. More particularly, both rotation of the plane of polarization and the phase shift of waves transmitted through the ferrite device of the type under consideration are functions of left and right hand circularly polarized phase constants, fully described in an article by M. G. Sakiotis and H. N. Chait, entitled, Ferrites at Microwaves, Proceeding I.R.E., vol. 41, pp. 87-93, Jan. 1953. With a positive coil current and right hand circular polarization, therefore, the phase is advanced while negative currents result in a phase delay. The term phase delay, of course, implies that the effective electrical length of the over-all phase shifter becomes longer due to the changes in the microwave permeability of the ferrite material. Conversely, left hand circular polarization input results in an output which is delayed in phase for positive currents and advanced in phase for negative currents. Since different values of phase shift are obtained for the positive and negative current the illustrative phase shifter is commonly referred to as a nonreciprocal device. It may also be noted that reversing the sense of the input circular polarization between right hand and left hand has the same effect upon the phase-current characteristics as reversing the coil current.

Referring next to the chart incorporated in FIG. 2, the theory of operation of the conventional Faraday rotator type ferrite phase shifter in the illustrative antenna system of FIG. 1 will be explained. Assuming that the energy fed to the individual antenna beam steering elements from the feed horn is right hand circularly polarized as indicated by the arcuate arrow 30 and the direction of propagation by arrow 32, a phase shift results in the first traversal of the waves through the ferrite rod 24. Reflection by the short circuit 28 results in a reversal of the sense of the polarization of the wave energy to left hand, as indicated by arrow 34, and the direction of propagation is also reversed as indicated by arrow 36. As previously stated this reversal of the sense of polarization is similar to a reversal of the polarity of the coil current. Hence, an additional phase shift 5 in the same sense as the first traversal is provided with the total resultant phase shift being doubled in value or 2gb and the energy is now left hand circularly polarized.

After target reflection a similar phase shift and reversal of sense of polarization will occur. It is evident, therefore, that when the phase shifter is used as a passive antenna beam steering element the correct or predetermined phase shift will be provided for only one sense of circular polarization if the magnetic field is maintained at the same polarity. In duplexing operations where the array antenna is utilized for both transmit and receive cycles it will then be necessary to reverse the magnetic field polarities to assist in the rejection of rain clutter in target definition. For a further explanation on this matter reference is directed to the text by M. I. Skolnik, Introduction to Radar Systems, McGraw-Hill Book Company, 1962, pp. 546-551. This problem then imposes a severe limitation upon the over-all operation and required ircuitry in the design of the antenna system with accompanying increase in cost and complexity.

In accordance with the teachings of the present invention a ferrite phase shifter which is inherently nonreciprocal may be operated in a reciprocal manner in an optically fed reflector type antenna array system and be polarization insensitive which results in the same value for phase shift for either right hand or left hand circular polarization. By reason of the polarization insensitivity the over-all phase shift will be independent of the coil current direction and consequently the device will be reciprocal from the standpoint of both current polarity and polarization. Referring to FIG. 3 the ferrite phase shifter of the present invention comprises a radiating element 38, a ferrite rod phase shifting section 40, and matching transformer section 42 and a short circuit 44 at the closed end of the over-all device. A selective reflector means 46 is disposed intermediate to the ferrite phase shifter section and the short circuited end to thereby impose a time delay on one orthogonal component of the circularly polarized wave while the remaining component is propagated in order that upon reflection by the short circuit the wave energy will have the same sense of polarization but an additional 180 phase shift as that traversing the ferrite phase shifter during the first traversal. The net phase shift provided by the reentrant device will be the same for either polarization of the microwave energy.

The selective reflector means preferably comprises a polarization inverter or circular polarizer such as a onequarter wavelength plate or a gridded structure whereby selective delay in phase by 90 (a quarter-wavelength) is applied to the circularly polarized electric vectors in a direction both down and back through the antenna element to result in an over-all delay of 180. Additionally, any anisotropic dielectric material having a dielectric constant of less than unity may be substituted for the quarter-wave plate or parallel grid structure with similar results. A structure comprising alternate layers of polystyrene and polyfoam is exemplary of such an anisotropic dielectric material.

Referring now to FIG. 4 the theory of the reciprocal polarization insensitive operation of my novel phase shifter incorporating a selective reflector and short circuit will be explained as presently understood. The coordinates of the graph are phase shift #1 along the vertical axis and current I along the horizontal axis. Curve 50 is a plot of phasecurrent characteristics of a right hand circularly polarized wave (RHP) and as previously noted for negative current values a phase delay is provided while for positive currents a phase advance. Curve 52 indicates the phase-current characteristics for left and circular polarization (LI-1P) with a phase advance for negative coil currents and a phase delay for positive currents. Assuming that the passive antenna element is provided with preset magnetic bias indicated by the 1 and dotted line 54, the reciprocal operation of the phase shifter may be described. If the incident energy polarization from the antenna horn is left hand circular, the forward traveling wave receives a phase delay & indicated by dotted line 56 during the first traversal through the ferrite phase shifter. Due to the disposition of the circular polarizer (quarter-wave plate) after the phase shifter section and before the short circuit, one orthogonal component will be unaffected while the mutually perpendicular one is speeded up. As a result the reflected wave from the short circuit or what may be referred to as the backward wave upon recombining of the components becomes a mirror image of the original wave or spatially oriented 180 out of phase with respect to the input polarization. Consequently, the wave now receives a phase advance Q51 in passing through the phase shifter the second time. In view of the fact that phase advance is simply negative phase delay the total phase shift after the reentrant path is simply a delay of or as indicated by the dotted line 58.

If the incident polarization is right hand circularly polarized and the same magnetic field I is maintained the phase advance after the first traversal will be 5 and after reflection and traversal through the circular polarizer section a phase delay of 5 is provided. We add the two phase shifts again to arrive at the same net value of dr and the device is completely reciprocal in that input right hand or left hand circularly polarized waves are received and radiated with the same net phase shift. Similarly the receiving cycle of the operation will be polarization insensitive and the phase shifter of the invention will work equally well with linearly polarized waves as well as elliptical waves since any of these waves can be duplicated by the sum of two circular orthogonal vectors. The phase shifter is therefore polarization insensitive and the net phase shift is independent of the coil current direction and consequently the disclosed device which is inherently a non-reciprocal structure may be operated in a reciprocal manner from the standpoint of both current polarity and polarization which is highly desirable in reflector type optically fed array antenna systems.

FIG. 5 is illustrative of an exemplary embodiment of the invention of a reentrant type phase shifter. The open end is enclosed by a radiating element 60, preferably of a dielectric material with the impedance characteristics required to transform the free space input and output electromagnetic energy for propagation within the ferritephase shifter section 62. The ferrite element 64 in the configuration of an elongated cylindrical rod of a pre-determined length in accordance with the frequency of operation and phase shift desired is positioned within the cylindrical waveguide 66 by means of a low-loss dielectric material 68, such as for example quartz. Other well known materials such as Teflon for supporting the ferrite ele' ment may be employed. The magnetic field producing means such as a solenoid 70 encirles the circular waveguide 66 and may consist of many thousand turns of a suitable conductive wire connected by means of leads 72 to an appropriate undirectional voltage source to provide the desired magnetic field bias. The selective reflector means 74 is disposed along the axis of the ferrite element and intermediate to this element and the short circuit 76 disposed at the closed end of the phase shifter. In the illustrative embodiment of the one-quarter wavelength conductive plate 78 is disposed angularly' within the outer conductive body member 80in such a manner as to change the spatial orientation of the horizonal and vertical electric field vectors of the electromagnetic energy traversing the phase shifter in both directions by the The plate member 78 is oriented diametrically at an angle of 45 in the manner of the circular polarization structures employed in wave transmission devices. A card or vane of a dielectric material may similarly be employed in the selective reflector section. In addition, any anisotropic dielectric material such as alumina maybe employed in this section. To assist in the matching of the Wave energy traversing the ferrite phase shifting section and the selective reflector section a matching transformer 82 of a dielectric material such as quartz is provided.

It should be understood that while the invention has been described with relation to circularly polarized energy, it will be equally applicable to any polarization of energy. Moreover, it is obvious to one skilled in the art that any number of selective reflector means may be employed for the purposes of the invention and the illustrative means are therefore not to be interpreted in a limiting sense in the preceding description.

What is claimed is:

1. A reentrant polarization insensitive electromagnetic wave energy phase shifter comprising:

a section of waveguide adapted to receive and propagate electromagnetic wave energy having fields polarized in a predetermined plane;

ferromagnetic means to produce a first predetermined value of electrical phase shift of energy passing through said waveguide in a forward direction;

conductive means enclosing one end of said waveguide to present a short circuit and return in a reverse direction substantially all energy incident thereon;

selective reflector means disposed intermediate to said short circuit means and said ferromagnetic means;

said reflector means defining structure having an electrical length of one-quarter of a wavelength of said energy to yield a 180 phase difference in the orthogonal distribution of the field-s of the reverse directed wave energy whereupon a second value of phase shift is introduced upon traversal through the ferromagnetic means;

the reflected energy having a total electrical phase shift of the difference between the first and second values and having the same sense of polarization orientation as the received energy.

2. A reentrant polarization insensitive electromagnetic Wave energy phase shifter comprising:

a section of circular waveguide adapted to receive and propagate circularly polarized energy;

a ferrite element supported along the longitudinal axis of said waveguide;

magnetic field producing means encircling said Waveguide adjacent to said ferrite element to provide a predetermined phase shift in the circularly polarized energy passing through said element in opposing directions;

said Waveguide being closed at one end to short circuit and reflect in a reverse direction substantially all energy incident thereon;

a polarization inverter including a one-quarter wavelength long conductive vane member diametrically disposed within such waveguide intermediate to the closed end and said ferrite element to produce a phase difference of 180 in the orthogonal distribution of the field vectors compositely defining said reflected circularly polarized waves whereupon a second value of phase shift is introduced upon traversal through the ferrite element;

the reflected output energy having a net electrical phase shift equal to the difference between the first and second values and having the same sense of polarization of the input circularly polarized wave energy.

3. A reentrant polarization insensitive electromagnetic wave energy phase shifter according to claim 2 wherein said polarization inverter comprises a body of an anisotropic dielectric material one-quarter of a Wavelength of said energy in length.

4. A device for providing a variable electrical phase shift of electrimagnetic wave energy comprising:

waveguide means for receiving and launching at one end said wave energy;

a ferrite element disposed along the longitudinal axis of said waveguide;

magnetic field producing means including an electric field coil concentrically wound around said waveguide in the region'of said ferrite element and a source of unidirectional current connected thereto to thereby alter the energy permeability of the ferrite element;

short circuiting means disposed at the opposing end of said waveguide to reflect substantially all energy incident thereon;

a fixed circular polarizing structure disposed intermediate to said short circuiting means and said ferrite element to introduce a phase difference in the orthogonal distribution of the electric field vectors of said energy between the point of exit after the first traversal through said ferrite element and the point of entry after deflection to traverse the ferrite element in the reverse direction.

5. A device according to claim 4 wherein said Waveguide section is circular.

6. A device according to claim 4 wherein said circular polarizing structure comprises a diametrically disposed vane member one-quarter of a wavelength of said energy in length.

7. A device according to claim 4 wherein said circular polarizing structure comprises an anisotropic dielectric material of predetermined length.

8. In a reflector type optically fed antenna system comprising:

means for generating and transmitting in free space electromagnetic wave energy;

means for collimating and directing said transmitted energy in a desired direction including an array of reentrant variable electrical elements;

each of said elements comprising a section of waveguide;

a radiating element enclosing one end of said waveguide to receive and launch said energy; ferromagnetic means to produce a predetermined electrical phase shift in energy traversing said waveguide in opposing directions, the value of phase shift being different for each direction to yield a net phase shift represented by the difference in said values;

waveguide shorting means enclosing the opposing end of said waveguide to reverse the direction of travel of said energy through said element;

selective reflector means disposed between said shorting and ferromagnetic means to introduce a 180 phase differential in one orthogonal electric vector of said energy relative to the allied mutually perpendicular vector and thereby invert the orientation of said vectors of said energy traversing the space defined between said shorting means and the adjacent end of said ferromagnetic means.

References Cited UNITED STATES PATENTS 8/1956 Fox 333-2.4.1 1/1965 Allen 333-241 ELI LIEBERMAN, Primary Examiner. 

8. IN A REFLECTOR TYPE OPTICALLY FED ANTENNA SYSTEM COMPRISING: MEANS FOR GENERATING AND TRANSMITTING IN FREE SPACE ELECTROMAGNETIC WAVE ENERGY; MEANS FOR COLLIMATING AND DIRECTING SAID TRANSMITTED ENERGY IN A DESIRED DIRECTION INCLUDING SAID TRANSMITTED ENENTRANT VARIABLE ELECTRICAL ELEMENTS; EACH OF SAID ELEMENTS COMPRISING A SECTION OF WAVEGUIDE; A RADIATING ELEMENT ENCLOSING ONE END OF SAID WAVEGUIDE TO RECEIVE AND LAUNCH SAID ENERGY; FERROMAGNETIC MEANS TO PRODUCE A PREDETERMINED ELECTRICAL PHASE SHIFT IN ENERGY TRAVERSING SAID WAVEGUIDE IN OPPOSING DIRECTIONS, THE VALUE OF PHASE SHIFT BEING DIFFERENT FOR EACH DIRECTION TO YIELD A NET PHASE SHIFT REPRESENTED BY THE DIFFERENCE IN SAID VALUES; WAVEGUIDE SHORTING MEANS ENCLOSING THE OPPOSING END OF SAID WAVEGUIDE TO REVERSE THE DIRECTION OF TRAVEL OF SAID ENERGY THROUGH SAID ELEMENTF SELECTIVE REFLECTOR MEANS DISPOSED BETWEEN SAID SHORTING AND FERROMAGNETIC MEANS TO INTRODUCE A 180* PHASE DIFFERENTIAL IN ONE ORTHOGONAL ELECTRIC VECTOR OF SAID ENERGY RELATIVE TO THE ALLIED MUTUALLY PERPENDICULAR VECTOR AND THEREBY INVERT THE ORIENTATION OF SAID VECTORS OF SAID ENERGY TRAVERSING THE SPACE DEFINED BETWEEN SAID SHORTING MEANS AND THE ADJACENT END OF SAID FERROMAGNETIC MEANS. 